Fix #112218: Do not require screen in object_join_poll

OBJECT_OT_object_join does not seem to use any screen/window during its
execution. Removing the relevant poll code so it makes it easier to run
this operator in the background.

Pull Request: https://projects.blender.org/blender/blender/pulls/112244

source/blender/editors/object/object_add.cc

@@ -4293,7 +4293,7 @@ static bool object_join_poll(bContext *C)
   }
 
   if (ELEM(ob->type, OB_MESH, OB_CURVES_LEGACY, OB_SURF, OB_ARMATURE, OB_GPENCIL_LEGACY)) {
-    return ED_operator_screenactive(C);
+    return true;
   }
   return false;
 }